As an example of a mobile communication system to which the present invention is applicable, a 3rd generation partnership project long term evolution (3GPP LTE) (hereinafter, referred to as LTE) communication system is described in brief.
FIG. 1 is a diagram schematically illustrating a network structure of an E-UMTS as an exemplary wireless communication system.
An evolved universal mobile telecommunications system (E-UMTS) is an advanced version of a legacy universal mobile telecommunications system (UMTS) and basic standardization thereof is currently underway in the 3GPP. E-UMTS may be referred to as an LTE system. For details of the technical specifications of UMTS and E-UMTS, see Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.
Referring to FIG. 1, E-UMTS includes a user equipment (UE), evolved Node Bs (eNode Bs or eNBs), and an access gateway (AG) which is located at an end of an evolved UMTS terrestrial radio access network (E-UTRAN) and connected to an external network. The eNBs may simultaneously transmit multiple data streams for a broadcast service, a multicast service, and/or a unicast service.
The UE is fixed or mobile. The UE is a device that transmits and receives user data and/or various kinds of control information though communication with a base station (BS). The term ‘UE’ may be replaced with ‘terminal equipment’, ‘Mobile Station (MS)’, ‘Mobile Terminal (MT)’, ‘User Terminal (UT)’, ‘Subscriber Station (SS)’, ‘wireless device’, ‘Personal Digital Assistant (PDA)’, ‘wireless modem’, ‘handheld device’, etc. A BS is typically a fixed station that communicates with a UE and/or another BS. The BS exchanges data and control information with a UE and another BS. The term BS' may be replaced with ‘Advanced Base Station (ABS)’, ‘Node B’, ‘evolved-Node B (eNB)’, ‘Base Transceiver System (BTS)’, ‘Access Point (AP)’, ‘Processing Server (PS)’, etc. In the following description, BS is commonly called eNB.
One or more cells are managed by one eNB. A cell is configured to use one of bandwidths of 1.25, 2.5, 5, 10, 15, and 20 MHz to provide a downlink (DL) or uplink (UL) transmission service to multiple UEs. Different cells may be configured to provide different bandwidths. The eNB controls data transmission and reception to and from a plurality of UEs. For DL data, the eNB transmits DL scheduling information to notify a corresponding UE of time/frequency resources through which the data is to be transmitted, coding, data size, and hybrid automatic repeat and request (HARQ)-related information. For UL data, the eNB transmits UL scheduling information to a corresponding UE to inform the UE of available time/frequency resources, coding, data size, and HARQ-related information. An interface for transmitting user traffic or control traffic between eNBs may be used. A core network (CN) may include an AG and a network node for user registration for the UE. The AG manages mobility of the UE on a tracking area (TA) basis, each TA including a plurality of cells.
Conventionally, the legacy LTE communication scheme mainly considers wireless communication between an eNB and a UE. However, demands for technology enabling direct communication between UEs have recently increased.
FIG. 2 is a conceptual diagram illustrating direct communication between UEs.
Referring to FIG. 2, UE-to-UE direct communication is performed between UE1 and UE2 and between UE3 and UE4. The eNB may control positions of the time/frequency resources, transmit power and the like for direct communication between UEs through a proper control signal. Direct communication between UEs is referred to as device-to-device (D2D) communication in the following description.
D2D communication has different requirements from legacy LTE communication in many aspects.